Both air and coil springs have a spring rate, but they are defined differently in each case.
A coil spring rate is defined as the weight needed to compress that spring one inch. This is counted in pounds in MTB suspension and is (almost) exclusively linear.
Linear means that the same force is required throughout the compression of a spring to compress through it’s stroke, remember stroke?
An air spring works differently. As the air spring is compressed the volume of the chamber holding the air gets smaller. Less space for the air equals an increase in pressure. This makes compressing that spring PROGRESSIVELY harder through it’s stroke, so an air shock is progressive.